See https://gist.github.com/mmp/21ea8ac6d7f682b6252b; scanlines seem to be written in reverse order as well. That test writes a 2-scanline-high (and 1 pixel wide) image EXR file and then reads the result with OpenEXR; the scanlines seem to be in reverse order..